This invention relates to a phosphor material, to a method of manufacturing the phosphor material, and also to a display device such as a vacuum fluorescent display and a field emission display.
In an electron beam excited display, emission is deteriorated if the fluorescent screen is charged up, so that electric charges accumulated on the fluorescent screen are required to be discharged. In the case of a cathode ray tube, a conductive film made of, for example, aluminum is formed on the back surface of the fluorescent screen (the surface facing the electron gun) to remove the electric charge. Since electron beam is accelerated to a high speed with an anode voltage of at least 10 kV, e.g., about 30 kV in general, the electron beam can pass through the conductive film to reach the fluorescent screen, whereby exciting phosphor. However, the vacuum fluorescent display and field emission display are designed to excite the phosphor with a low voltage electron beam which is accelerated with an anode voltage of 1 kV or less. Therefore, if a conductive film is formed on the back surface of the fluorescent screen in such a low voltage electron beam excited display, the electron beam is incapable of passing through the conductive film. As a result, the phosphor cannot be sufficiently excited. Accordingly, the conductive film may not be applied to the low voltage electron beam excited display.
In order to overcome the above problem, there has been proposed to employ a phosphor material provided with conductivity for a low voltage electron beam excited display so as to prevent the fluorescent screen from being charged up.
As one example of such an attempt, a phosphor exhibiting conductivity has been developed. For example, green-emitting ZnO:Zn phosphor is electrically conductive. However, a conductive phosphor emitting light other than green and exhibiting sufficient luminance has not been developed as yet. Therefore, it is impossible to manufacture a color display by making use of conductive phosphors.
On the other hand, there has been studied to employ a phosphor material consisting a mixture comprising a phosphor and a conductive material. For example, there has been proposed to employ a phosphor and In.sub.2 O.sub.3. However, since a mixing ratio of the phosphor is lowered in such a phosphor material, the luminous efficiency is inevitably deteriorated.
Further, a phosphor material comprising ZnS:Ag,Cl covered with a conductive layer consisting of In.sub.2 O.sub.3 prepared by sol-gel method is disclosed in a document, "Extended Abstracts of the 2nd International Conference on the Science and Technology of Display Phosphors, Nov. 18 (1996), p.319. However, it is assumed that a phosphor particle employed in this method is of nearly polyhedron in shape, which is synthesized by ordinary flux method. Accordingly, even if the phosphor particle is covered with an In.sub.2 O.sub.3 conductive layer, the polyhedron shape would be still maintained. When a phosphor layer is formed by making use of such a polyhedral phosphor material, it is difficult to form a phosphor layer of high density due to the poor packing property of the polyhedral phosphor material, thus making it disadvantageous in terms of luminance. Furthermore, a high electric field may be applied to the projected portions of the rough fluorescent screen, thus making it difficult to obtain a uniform emission.